Recently, small electronic appliances such as home video cameras, note PCs, and smart phones have become widespread, and attaining higher capacity and longer service life of batteries has become a technical problem.
Given that hybrid vehicles, plug-in hybrid vehicles, and electric vehicles will be further spread, size reduction of batteries is also a technical problem.
At present, graphite-based negative electrode active materials are utilized for lithium ion batteries. However, graphite-based negative electrode active materials have technical problem as described above.
Accordingly, alloy-based negative electrode active materials have gained attention, which have higher capacity than those of the graphite-based negative electrode active materials. As an alloy-based negative electrode active material, silicon (Si)-based negative electrode active materials and tin (Sn)-based negative electrode active materials are known. To realize a lithium ion battery having a smaller size and a longer life, various studies have been conducted on the above described alloy-based negative electrode active materials.
However, an alloy-based negative electrode active material repeatedly undergoes large expansion and contraction in volume at the time of charging/discharging. For that reason, the capacity of the alloy-based negative electrode active material is prone to deteriorate. For example, a volume expansion/contraction rate of graphite associated with charging is about 12%. In contrast, the volume expansion/contraction rate of Si single substance or Sn single substance associated with charging is about 400%. For this reason, if a negative electrode plate of Sn single substance is repeatedly subjected to charging and discharging, significant expansion and contraction occur, thereby causing cracking in negative electrode compound which is applied on the current collector of the negative electrode plate. Consequently, the capacity of the negative electrode plate sharply decreases. This is chiefly caused by the fact that some of the active substances are freed due to volume expansion/contraction and thereby the negative electrode plate loses electron conductivity.
US2008/0233479A (Patent Literature 1) proposes a method for solving the above described problem of an alloy-based negative electrode active material. To be specific, the negative electrode material of Patent Literature 1 includes a Ti—Ni superelastic alloy, and Si particles formed in the superelastic alloy. Patent Literature 1 describes that a large expansion/contraction change of Si particle which occur following occlusion and release of lithium ions can be suppressed by a superelastic alloy.
However, it is questionable that the technique disclosed in Patent Literature 1 sufficiently improves the charge-discharge cycle characteristics of the secondary battery. Most of all, it may be highly difficult to actually produce the negative electrode active material proposed by Patent Literature 1.